1. Field of the Invention
This invention relates to a tape recorder employing DC erasing system. More particularly, it concerns a tape recorder which is switchable to an AC erase unit.
2. Description of the Prior Art
Two methods are used for erasing signals recorded on a magnetic tape of tape recorder, namely, the DC and AC erasing methods. In the former, DC erasing current is applied to the erase head and, in the latter, AC erasing current is used.
Of these, the DC method is simple and, if the erase head itself consists of a permanent magnet, avoids the need for a power supply for the DC erasing current and results in zero power consumption. Therefore, it is often used in portable tape recorders energized with batteries.
FIG. 1 is a presentation of an example of the tape recorder employing such DC erasing method. Here, a microphone 1 is connected to fixed contact REC of a recording and replay changeover switch 3A through an amplifier 2. This changeover switch 3A is connected a terminal of a recording and replay head 6 through its fixed contact PLAY and through a regeneration amplifier 7. Also, its movable contact S is connected to another terminal of the recording and replay head 6, through a recording amplifier 4 and bias trap 5. In addition, the moving contact S is connected to one terminal of a speaker 11, through a variable resistor 8, power amplifier 9 and earphone jack 10. The other terminal of the speaker 11 is connected to the fixed contact PLAY of a selector or changeover switch 3C. The selector switch 3C is grounded at the moving contact S. In this case, the switch 3C is ganged with the changeover switch 3A.
The top terminal of the recording and replay head 6 is connected to the fixed contact REC of a changeover switch 3B. The other terminal of head 6 is connected to the fixed contact PLAY of said changeover switch 3B. This changeover switch 3B is grounded at its moving contact S. In this case, this changeover switch 3B is ganged with the changeover switch 3A.
The junction between the recording and replay head 6 and the fixed contact PLAY of the changeover switch 3B is connected to a bias oscillator circuit 12. This bias oscillator circuit 12 is further connected to the moving contact S of the changeover or selector switch 3D. This switch 3D in turn connected to the DC power supply Vcc from its fixed contact REC. The selector switch 3D is also ganged with the changeover switch 3A.
The foregoing selector switch 3D is connected to an erase head 13 at its moving contact S. The erase head 13 is supplied with DC erasing current via said selector switch 3D.
Tape changeover switches 141, 142, and 143 are switched between normal tape or metal tape positions depending on kind of tape used.
Now suppose this tape recorder is set to tape replay. Then, the moving contact S of each of switches 3A, 3B, 3C and 3D is switched to the fixed contact PLAY. (FIG. 1 indicates this condition.) Also, the recording and replay head 6 is connected to the speaker 11 by the changeover switch 3A. Further, the switch 3B grounds the junction point of the recording and replay head 6, bias trap 5 and bias oscillator circuit 12. Then, the changeover switch 3C also grounds the other terminal of the speaker 11. In addition, the changeover switch 3D blocks a DC power supply to the bias oscillator circuit 12 and erase head 13. When replay output is generated from the head 6 on this condition, the output is amplified by the regenerating amplifier 7. Thereafter, the output is further amplified in the power amplifier 9, then it is generated in the speaker 11 as replay sound. The replay sound volume is adjustable by the variable resistor 8. Moreover, if an earphone is connected to the earphone jack 10, replay sound can be heard from the earphone instead of the speaker 11.
Next, suppose the tape recorder is set to recording condition. Then, moving contacts S of changeover or selector switches 3A, 3B, 3C and 3D are switched to the fixed contacts REC. Thus, the microphone 1 is connected to the recording and replay head 6 by means of changeover switch 3A. In turn, a junction point between the head 6 and replay amplifier 7 is grounded with the changeover switch 3B. Thus, the speaker 11 is disconnected by the changeover switch 3C. In addition, changeover switch 3D connects the DC power supply Vcc to the bias oscillator circuit 12 and erase head 13. If an input enters the microphone 1, e.g. voice sound, the output of the microphone 1 is amplified by the microphone amplifier 2. Then, it is supplied to the recording amplifier 4 and transmitted to the head 6 as recording signal current. At the same time, AC bias current is applied to the head 6 from the bias oscillator circuit. Under these circumstances, the output of the microphone 1 is continuously recorded on magnetic tape from the head 6. In this case, DC erasing current is supplied to the erase head 13 from the DC power supply to erase signals previously recorded on the tape.
Accordingly, if such kind of tape recorder is used, recording and replay can be done sequentially.
Unfortunately, however, such a DC erasing method conventionally used for these tape recorders utilizes a demagnetizing method which applies a saturated DC magnetic field to the magnetic layer of the tape to demagnetize it to a certain level of residual magnetic field. In consequence, if there is any irregularity in the magnetic layer of the tape or uneven contact of the head to the tape, scattering may be caused in the residual magnetic flux density. This sometimes causes low-quality recording or replay. Worse, in some cases, it produces replay noise, while significantly aggravating defects in replay sound. Another shortcoming of the DC erasing method is that it generated larger signal distortion of the recorded and the replayed sound than the AC erasing method.
Therefore, there has been a general tendency for tape recorders to make only limited use of DC erasing method for a narrow range of purposes, e.g. conversations, etc., because of its many inherent defects.